This invention relates to radio receivers and more particularly to circuitry which reduces the noise level of radio receivers.
It is known to attenuate or block transmission to the speaker of a radio when the signal received by the radio receiver drops and thus to eliminate the sound of white noise from the speaker. It is also known in some applications, to reduce irritating high frequency noise by reducing the bandwidth of signals applied to a speaker such as in phonograph players and thus eliminate the more irritating portions of the high frequency noise.
In prior art radio circuits, a squelch circuit completely blocks noise when the received signal falls below a threshold value set at a level which would result in an incomprehensible audio signal being applied to the speaker. While filtering has long been used to remove noise, it has not been adapted in a way that removes noise-ups.
The prior art circuits have a disadvantage in that portable receivers and particularly those used in wireless microphones are often operated under conditions in which the received signal is not so low as to cause the squelch circuit to block all audio to the speaker but is sufficiently low so that substantial irritating white noise is passed to the speaker. This problem is particularly aggravated in wireless microphones because the receiver is intermittently carried into areas where fading may occur and may be repeatedly turned on and off while in such areas, resulting in frequent irritating noise-ups.